1. Field of the Invention
This invention relates to an automatic focus adjusting device for use in a camera or the like.
2. Related Background Art
Many of the automatic focus adjusting systems of single-lens reflex cameras have been designed to focus to an object by repetitively effecting the cycles of "focus detection (sensor signal inputting and focus detection calculation) and lens driving". The amount of lens driving in each cycle is based on the defocus amount at a point of time whereat focus detection has been effected in that cycle, and this presumes that the defocus amount during focus detection is eliminated at the end of lens driving.
As a matter of course, focus detection and lens driving require their respective times, and in the case of a stationary object, the defocus amount does not vary as long as the lens is not driven and therefore, the defocus amount to be eliminated at a point of time whereat lens driving has been completed is equal to the defocus amount at a point of time whereat focus detection has been effected, and correct focus adjustment is accomplished.
However, in the case of an object in great motion, the defocus amount varies during focus detection and lens driving, and sometimes, said defocus amount to be eliminated and the detected defocus amount differ remarkably from each other and this results in the problem that the lens is not in focus to the object at the end of lens driving.
The same assignee of the present application has previously filed Japanese Patent Application No. 62--263728 as an automatic focus adjusting system directed to the solution of the above-noted problem.
The gist of the method disclosed in the above-noted patent application is to foresee the variation in the defocus attributable to the movement of an object, and to apply correction to the amount of lens driving (hereinafter referred to as the pursuit correction) in view of the detected variation in the defocus in each said cycle and the time interval between said cycles, and the solution of the above-noted problem by this method is expected from the viewpoint of the focusing accuracy at the end of lens driving.
However, when the pursuit correction is actually done, the following problem arises.
If the object in the distance measuring field shifts to another object when the object is being pursued in the pursuit correction mode, the continuity of the change in the imaging plane position will be lost and therefore, if foreseeing is done on the basis of the data of the past object and the data of the new object, the foreseeing will become wrong and as a result, the lens will be driven to an entirely different position.
If the object in the distance measuring field thus shifts to another object, wrong foreseeing will be done, and there exists the problem that this is not eliminated as long as foreseeing control is effected by the use of the data of the old object.
The operation of the device of the above-mentioned Japanese patent application will hereinafter be described with reference to the accompanying drawings.
FIG. 2 is a graph for illustrating the lens driving correction method proposed by the above-mentioned Japanese patent application. In FIG. 2, the horizontal axis represents time t and the vertical axis represents the imaging plane position x of the object.
The curve x(t) indicated by the solid line represents the imaging plane position at time t of an object which comes near the camera in the direction of the optic axis when the photo-taking lens is at infinity. The curve l(t) indicated by broken line represents the position of the photo-taking lens at the time t, and the lens is in focus when x(t) and l(t) coincide with each other. [ti, ti']represents the focus detecting operation, and [ti, ti+1]represents the lens driving operation. In the example shown in FIG. 2, it is assumed that the imaging plane position changes in accordance with a quadratic function. That is, if the current and past two imaging plane positions (t.sub.1, x.sub.1), (t.sub.2, x.sub.2) and (t.sub.3, x.sub.3) are known at time t.sub.3, the imaging plane position x.sub.4 at time t.sub.4 after TL (AF time lag + release time lag) from the time t.sub.3 can be foreseen on the basis of the equation x(t)=at.sup.2 +bt+c.
However, what can actually be detected by the camera are not the imaging plane positions x.sub.1, x.sub.2 and x.sub.3, but the defocus amounts DF.sub.1, DF.sub.2, DF.sub.3 and the amounts of lens driving DL.sub.1, DL.sub.2 as converted into the amounts of movement of the imaging plane. Time t.sub.4 is a value in the future to the last, and is a value which varies as the accumulation time of an accumulation type sensor is varied by the luminance of the object, and here, for simplicity, it is assumed as follows: EQU t.sub.4 -t.sub.3 =TL=TM.sub.2 +(release time lag) (1)
Under the above assumption, the amount of lens driving DL.sub.3 calculated from the result of the focus detection at time t.sub.3 can be found as follows: EQU x(t)=at.sup.2 +bt+c (2)
When (t.sub.1, l.sub.1) in FIG. 2 is considered to be the origin, ##EQU1##
If the equations (3), (4) and (5) are substituted into the equation (2), a, b and c are found as follows: ##EQU2##
Consequently, the amount of lens driving DL.sub.3 as converted into the amount of movement of the imaging plane at time t.sub.4 is found as follows: ##EQU3##
A problem arising when the object in the distance measuring field shifts to another object will now be described with reference to FIG. 3.
FIG. 3 shows the relation between time and the imaging plane position, and the solid line indicates the imaging plane position of a first object, and the dot-and-dash line indicates the imaging plane position of a second object.
Here, let it be assumed that at times t.sub.1 and t.sub.2, focus detection is effected for the first object and the lens is driven, and at time t.sub.3, focus detection is effected for the second object.
Thereupon, on the camera side, the imaging plane positions x.sub.1, x.sub.2 and x.sub.3 ' at times t.sub.1, t.sub.2 and t.sub.3, respectively, are calculated from the defocus amount and the amount of lens driving obtained by focus detection, and a quadratic function f(t) passing through (t.sub.1, x.sub.), (t.sub.2, x.sub.2) and (t.sub.3, x.sub.3 ') is calculated, and from this f(t), the imaging plane position x.sub.4 " at time t.sub.4 is foreseen.
However, the imaging plane position of the first object at the time t.sub.4 is x.sub.4 and the imaging plane position of the second object at the time t.sub.4 is x.sub.4 ', and x.sub.4 " obtained by foreseeing is a position differing from the imaging plane position of either object.
Thus, to foresee the imaging plane position x.sub.4 of the first object, it is necessary to find a function passing through (t.sub.1, x.sub.1), (t.sub.2, x.sub.2) and (t.sub.3, x.sub.3), and to foresee the imaging plane position x.sub.4 ' of the second object, it is necessary to find a function passing through (t.sub.1, x.sub.1 '), (t.sub.2, x.sub.2 ') and (t.sub.3, x.sub.3 ').
However, the distinction between the first object and the second object is not made in the camera and therefore, foreseeing calculation is effected by the use of the defocus amount obtained at the time t.sub.3 by focus detection. As a result, the foreseeing function is neither the approximate function of the imaging plane position of the first object, nor the approximate function of the imaging plane position of the second object, and thus, the foreseen lens driving position also becomes wrong. This is a problem which arises whenever the photographer changes over the main object to the second object while pursuing the first object because the wrong foreseeing as described above will result if the data of the focus detection effected for any other object than the main object exists in the data used for foreseeing.
The assignee of the present invention has previously filed Japanese Patent Application No. 62--328233 as a countermeasure for such a problem. The gist of this patent application is to immediately discontinue the pursuit mode by said foreseeing process when there is brought about a condition unsuitable for foreseeing, such as the loss of the continuity of the change in the imaging plane position or the low luminance of the object, thereby preventing said inconvenience, and according to the technique shown in this patent application, the above-described inconvenience can be prevented.
As described above, according to the technique shown in the aforementioned patent application, when an object differing from the original object is distance-measured, foreseeing driving can be inhibited and the out-of-focus problem as noted above can be solved, but even when there is camera shake or another object temporarily crosses the front of the camera, said foreseeing driving is inhibited and much time is taken until another foreseeing driving is started, and this leads to the undesirable possibility of missing the shutter opportunity.